This invention pertains to the art of testing specimens for resistance to deterioration due to sunlight, and more particularly to a materials test chamber using discharge lamps such as xenon lamps for the light source.
This invention is particularly applicable to an accelerated weathering apparatus employing xenon lamps to simulate the deterioration of specimens caused by sunlight, and will be described with particular reference thereto. However, it will be appreciated that the application has broader applications and may be advantageously employed in connection with other lamps and devices.
U.S. Pat. No. 5,488,267, to Rudolph et al., discloses an illumination system for weathering and fading resistance test instruments. Provided is a pulse-driven xenon gas discharge lamp with an elongated discharge lamp that has an electrode spacing in a range from 10-50 cm. The duty factor of the lamp current is adjusted by means of an electronic control unit.
U.S. Pat. No. 4,747,645, to Rudzki discloses a weathering and fading resistance test instrument which provides for adjustment of the defined radiation spectrum that encompasses ultra-violet (UV) radiation, and infra-red (IR) radiation, by means of a xenon radiation source. Two sectors are provided for the radiation. The first sector includes UV mirrors that are impermeable to UV radiation, and a second sector includes a UV filter and at least one IR filter that are permeable to visible light. Each one of the sectors is associated with a corresponding xenon radiation source. The filter/radiation system is initially surrounded by a quartz inner cylinder with a selectively reflective layer for IR that is permeable to UV light, and an adjacent water jacket which absorbs longwave IR radiation, then by a quartz outer cylinder, and finally by a three-piece sealing jacket made of glass. This configuration is intended to dampen the intensity of a given spectral component in a targeted fashion, or to vary and set the ratio of the ultra-violet to the infra-red components.
Xenon lamps are known to operate efficiently at high-current density. Higher currents produce more light output per watt of electrical input. It is also believed that high current density also changes the shape of a Spectral Power Distribution (SPD) to produce more UV and less IR as a percent of total output. Such a result is considered to be useful for weathering testers.
In existing 50/60 hertz ballasts (or DC ballasts), high current is known to also produce extra wattage (heat) on the xenon lamp. This reduces the xenon lamp life, which is already known to have a substantially short life span. Furthermore, high current raises the total wattage consumption. Rudzki attempts to protect the xenon lamp by providing water cooling. It is noted that even with this water cooling protection, such machines will still have high wattage consumption. The system of Rudolph et al., describes an amplitude discharge current in the range of between 15 and 100 amps at a cold fill pressure of less than 400 mbar. The duty factor of such a device is from 1:1 to 1:100, and in a preferred embodiment the discharge current comprises pulses of alternating polarity, with the maximum duration of the current pulse noted as being 10 ms.
A drawback of existing devices, as noted above, is the rate of deterioration of the xenon lamp and the increased operation cost due to high power consumption. In view of this, existing accelerated weathering apparatuses which employ xenon discharge lamps have not been able to fully exploit the benefits which would be available by operating the xenon lamps at high current densities. The benefits of such operation xenon lamps include efficient operation, the production of more UV with less IR as a percentage of total lamp output.
Therefore, it has been deemed desirable to find a manner of operating an accelerated weathering apparatus employing a xenon lamp at very high current density. It is further desired that such operation does not shorten xenon lamp life, and allows for acceptable power consumption for weathering apparatuses employing xenon lamps.
The present invention contemplates a new and improved accelerated weathering apparatus that overcomes all of the above-noted problems and others, and provides for operation of a xenon lamp at a high level of efficiency.
In accordance with one aspect of the present invention, an accelerated weathering apparatus includes a test chamber having a support member to support a specimen which is to be tested. A lamp produces light within the test chamber. A high instantaneous, low average current ballast controls operation of the lamp. The ballast generate current pulses, at least some of the current pulses having a peak value of at least 100 amps and root mean square (rms) average values of less than ⅕ of the peak value over a full signal.
In accordance with a more limited aspect of the present invention, the high instantaneous, low average current ballast includes a front end, which is supplied by an external power source, for generating high DC voltage. An arc capacitor is connected to store energy developed from a charging action of the front end and to selectively release the arc capacitor energy into the lamp as short, high current pulses. A starting transformer generates a voltage used to ionize gas within the lamp to strike an arc. A switch is connected between the arc capacitor and the lamp for controlling generation of the current pulses. An internal or external timer controls the switch and transformer for selectively generating and supplying current pulses to the lamp.
In accordance with a more limited aspect of the present invention, a dimmer limits energy transfer from the capacitor to the lamp.
In accordance with another aspect of the present invention, a method of operating a high instantaneous, low average current ballast for controlling a xenon lamp includes generating a high DC voltage to charge a capacitor to a desired level. The charge capacitor is connected to the xenon lamp through a switch and a starting transformer, which generates a voltage field at the xenon lamp when fired, thereby ionizing gases within the xenon lamp. A main current path is formed, where the main current path includes the charged capacitor, the switch, and the xenon lamp. The switch is disposed between and in series with the charge capacitor and the xenon lamp. Current is delivered to the xenon lamp through the main current path and the switch is opened such that the capacitor is again disconnected from the xenon lamp and charged up to the desired value.
In accordance with a more limited aspect of the present invention, the current delivered to the xenon lamp takes form in current pulses, at least some of the current pulses having a peak value of at least 100 amps and a root mean square (rms) average value of less than ⅕ of the peak value over a full signal.
A principle advantage of the invention is an accelerated weathering apparatus that operates at a high efficiency using a xenon lamp, wherein the xenon lamp does not overheat, and whose life expectancy is not diminished.
Another advantage of the invention resides in the improvement in the ratio of UV to IR by operating the xenon lamp at a high current level.
Still another advantage of the invention is realized by maintaining a low overall average current supplied to the xenon lamp.
Still other advantages and benefits of the invention will become apparent to those skilled in the art upon a reading and understanding of the following detailed description.